Permafrost is soil or rock that remains below 0°C / 32°F throughout the year, and forms when the ground freezes sufficiently in winter to produce a frozen layer that lasts all through the following summer.

Additional vast areas of land also undergo seasonal freezing and thawing, but if the ground thaws in the summertime, it's not permafrost.

This continually "frozen ground" exists in approximately one-quarter of the exposed land surface of the Northern Hemisphere. That's about 23 million square kilometers / 9 million square miles! It can even reach out under the seabed of the shallow shelf seas along the coasts of the Arctic Ocean.

Permafrost occurs in more than 50% of Russia and Canada, 82% of Alaska, 20% of China, and probably all of Antarctica. In Siberia, permafrost can reach depths of up to 5,000 feet / 1525 meters. In northern Alaska the maximum depth is about 2,000 feet / 600 meters.